The present invention relates to mask fabrication and particularly to a control system used in mask fabrication for controlling feature critical dimensions (CD).
The critical dimension, or absolute size of a feature, includes linewidth, spacing or contact dimensions. Errors in mask critical dimensions have great impact during image transfer to a wafer. Only a mask with correct critical dimensions can correctly transfer an image onto a wafer. As optical lithography pushes to smaller and smaller dimensions, patterned features smaller than the wavelength of light must be routinely manufactured. In this system mask errors make up an increasingly large share of the sources of critical dimension (CD) errors. Critical dimension control is affected by a variety of factors including fabrication tools and equipment, recipes, and raw materials.
FIG. 1 is a schematic view showing a conventional mask fabrication process. A conventional mask fabrication process includes writing, baking, developing, etching, after-etch inspection (AEI), re-etching, stripping of the photoresist, and after-strip inspection (ASI) steps. Critical dimensions are largely determined by specific characteristics of the aforementioned steps. Accordingly, the process parameters of the writer and etcher are largely determined by device data of the processed mask and material data of the raw materials used in the writing step. The inspection results obtained by the after-etch inspector and after-strip inspector are not utilized to fine-tune the processing conditions in the conventional mask fabrication systematically.
The greatest disadvantage of the conventional mask fabrication process is that it is not effective in controlling CD variations. There is usually a substantial error rate in critical dimensions subsequent to the first etching step such that a re-etch step is needed. The re-etch step is costly and impacts the mask uniformity.
Hence, there is a need for a mask fabrication process that addresses the problems arising from the existing technology.